In general, image digitization systems use a low pass filter to limit the noise bandwidth and to eliminate possible aliasing. The use of the low pass filter, however, typically effects strong attenuation of the frequency close to the Nyquist frequency, thus introducing some smearing of sharp edges of the image. When a video signal is the output of a D/A converter that converts both image and alphanumeric information, or the output of a CCD camera, the intrinsic bandwidth of the signal extends well beyond the D/A sampling frequency. In that case, if oversampling is to be avoided, when the signal is redigitized the signal can be digitized synchronously, i.e., using an A/D converter clock that operates at the same frequency as the D/A converter clock. If that is done, aliasing may be prevented. But then in order to preserve the sharpness of the alphanumeric information, the front end circuit should not filter the signal. In this case, however, the bandwidth of the noise is not limited and thus causes noise in the redigitized image, specifically visible in its flat field areas.
Separately, image digitization systems which contain conventional two hundred and fifty-six (2.sup.8) true gray levels must be redigitized in at least 2.sup.9 gray levels to remove the intrinsic uncertainty of the least significant bit, which would otherwise result in some artifacts specifically visible in a flat field area of the image: specifically, when the image must be compensated for high gamma correction effects, for example. In dealing with the digitization of high resolution images (1280 lines .times. 1024 pixels/line) that are displayed in progressive mode at 60 Hz or in interlaced mode at 120 fields per second, pixel time approaches eight or nine nanoseconds, which corresponds to a D/A converter rate of approximately 110 MHz for synchronous digitization (and much more, two or three times that, if oversampling is applied). However, nine or ten bit A/D converters that would operate at such speeds would be expensive and difficult to build. Such high speed data flow typically requires an intermediate memory buffer (frame grabber or line buffers) for interfacing with a conventional main memory that typically has a much lower throughput.